Many battery-powered portable electronic devices, such as laptop computers, Portable Digital Assistants, cell phones, and the like, require memory devices that provide large storage capacity and low power consumption. To reduce the power consumption and thereby extend the battery life in such devices, the devices typically operate in a low-power mode when the device is not being used. In the low-power mode, a supply voltage or voltages applied to electronic components such as a microprocessor, associated control chips, and memory devices are typically reduced to lower the power consumption of the components, as will be appreciated by those skilled in the art. Although the supply voltages are varied to reduce power consumption in the low-power mode, data stored in the electronic components such as the memory devices must be retained.
Because large storage capacity is typically desired to maximize the amount of available storage in portable devices, it is typically desirable to utilize dynamic random access memory (DRAM), which has a relatively large storage capacity, over other types of memories such as static random access memories (SRAM) and non-volatile memories such as FLASH memory. In a DRAM, the data is “dynamic” because the data stored in memory cells in the DRAM must be periodically recharged or “refreshed” to maintain the data, as will now be explained in more detail with reference to FIG. 1. FIG. 1 illustrates a portion of a conventional DRAM memory-cell array 100 including a plurality of memory cells 102 arranged in rows and columns, one of which is shown in FIG. 1. The memory cell 102 includes an access transistor 104 and a storage capacitor 106 connected in series between a digit line DL and a reference voltage VCC/2. The storage capacitor 106 includes a first conductive plate 107 coupled to the access transistor 104 and a second conductive plate 109 coupled to the reference voltage VCC/2.
A word line WL activates the access transistor 104 in the memory cell 102, and also activates the access transistors of all other memory cells (not shown) contained in the same row of the array 100 as the memory cell 102. To write data into the memory cell 102, a sense amplifier 108 drives the digit line DL and a complementary digit line DL* to complementary voltage levels corresponding to the data to be stored in the memory cell. The word line WL is then activated, turning ON the access transistor 104 and transferring charge through the access transistor to charge the storage capacitor 106 to the voltage level on the digit line DL corresponding to the data to be stored. The word line WL is thereafter deactivated, turning OFF the access transistor 104 and isolating the storage capacitor 106 from the digit line DL to thereby store the data in the form of a voltage across the storage capacitor.
To read data from the memory cell 102, the sense amplifier 108 equilibrates the digit lines DL, DL* to a predetermined voltage level and thereafter activates the word line WL to turn ON the access transistor 104. In response to the access transistor 104 turning ON, charge is transferred between the storage capacitor 106 and the digit line DL, causing the voltage on the digit line DL to be slightly higher or lower than the voltage on the digit line DL*. The sense amplifier 108 senses the difference between the voltages on the digit lines DL and DL* and drives the voltages on the digit lines to complementary levels in response to the sensed difference. For example, assume a voltage VCC/2 corresponding to a binary 1 is stored across the capacitor 106. In this situation, when the access transistor 104 is activated the equilibrated voltage on the digit line DL will increase slightly relative to the equilibrated voltage on the digit line DL*. As a result, the sense amplifier 108 will drive the voltage on the digit line DL to a supply voltage VCC and will drive the complementary digit line DL* to a reference voltage. The complementary voltages on the digit lines DL, DL* thus correspond to the data stored in the memory cell 102, and the sense amplifier 108 thereafter applies these signals to other circuitry (not shown) to thereby provide the circuitry with the data stored in the memory cell.
As previously mentioned, the data stored in the memory cell 102 in the form of the voltage across the capacitor 106 must be periodically refreshed. This is true because once the data is stored in the form of a voltage across the capacitor 106 and the access transistor 104 is deactivated, leakage currents ILK result in this stored voltage changing over time and, if not refreshed, may result in a different binary state of data being stored in the memory cell. These leakage currents ILK arise, for example, from the flow of charge stored on the conductive plate 107 of the capacitor 106 through the access transistor 104 even when the access transistor is turned OFF, and may also arise from the flow of charge from the conductive plates 107, 109 to ground, as well as the flow of charge from the plate 107 through a dielectric (not shown) to the plate 109, as will be appreciated by those skilled in the art. From the above description of the conventional DRAM memory cell 102, it is seen that each time data is read from the memory cell the storage capacitor 106 is again charged to the proper voltage corresponding to the data stored in the cell. Thus, to refresh memory cells 102, the memory cells are merely accessed as in a read operation with the sense amplifier 108 driving digit lines DL, DL* to complementary voltages corresponding to the data stored in the memory cell and thereby charging the storage capacitors 106 to the proper voltage.
The rate at which the data restored in the memory cells 102 must be periodically refreshed is known as the refresh rate of the cells, and is a function of a number of different parameters, including the operating temperature of the DRAM containing the array 100, the number of rows of memory cells in the array, and the value of the supply voltage VCC applied to the DRAM, as will be appreciated by those skilled in the art. For example, if the array 100 includes N rows of memory cells 102 and each memory cell must be refreshed every M milliseconds, the refresh rate is M/N milliseconds/row, meaning that one row must be accessed every M/N milliseconds in order to properly refresh the memory cells, with every row being accessed at least once every M milliseconds. As the supply voltage VCC decreases, the refresh rate increases due, for example, to a reduced voltage being stored across the storage capacitors 106 and the need to refresh this voltage more frequently to ensure the stored voltage does not decay to an insufficient level due to the leakage currents ILK. The refresh rate also must increase as the supply voltage VCC decreases due to the possibility of restoring incorrect data into the memory cell 102, as will be appreciated by those skilled in the art.
When the memory-cell array 100 is contained in a DRAM, a memory controller typically reads data from desired memory cells 102 in response to requests from a microprocessor or other control circuit, each accessed memory cell being automatically refreshed as previously described. The data stored in all the memory cells 102 and not just those accessed by the memory controller, however, must be periodically refreshed. As a result, during normal operation the memory controller will periodically apply a refresh command to the DRAM containing the array 100, causing control circuitry (not shown) to access each memory cell 102 as previously described and thereby refreshing the memory cells. Even when the memory controller is not accessing the DRAM, the memory cells 102 must still be periodically refreshed. To refresh the memory cells 102 in this situation, the memory controller applies a self-refresh command to the DRAM, placing the DRAM in a self-refresh mode of operation during which circuitry internal to the DRAM (not shown in FIG. 1) refreshes the memory cells 102 periodically, as will be appreciated by those skilled in the art.
As previously described, in portable and other electronic devices containing DRAM, the supply voltage VCC applied to the DRAM is typically reduced during a low-power mode of operation to reduce power consumption and extend battery life of the device. Notwithstanding the reduced supply voltage VCC, the memory cells in the DRAM must be adequately refreshed to ensure the integrity of the stored data. There is a need for an improved circuit and method for controlling the refresh rate of dynamic data stored in a DRAM or other integrated circuit when the supply voltage is reduced to a very low level during a low-power mode of operation.